A variety of vacuum tubes are designed to produce a light output by converting a flow of electrons into photons at an output screen. Typically, the light output forms an image, displays a signal or signals an event, i.e., acts as a detector or counter. Well known examples of such tubes include cathode ray tubes, x-ray image intensifier tubes, infrared detectors and night vision tubes. As used in this description the term light output is intended to include the output of photons in the infrared region of the electromagnetic spectrum.
The input signal in many types of such tubes also consists of photons which are converted into electrons at a photocathode and amplified before being converted back to photons at the output screen. Such tubes may be designed for sensitivity to input photons which are in a different region of the spectrum than the output photons.
Certain image intensifiers and detector tubes now employ input photocathodes comprising Gallium Arsenide (GaAs) or other III-V semiconductor compounds. While photocathodes comprising III-V semiconductor compounds offer advantages which make them indispensable in certain applications, they are particularly susceptible to contamination by ions and neutral gas molecules which may be released in the vacuum tube during normal operation. Since the photocathode is negatively biased, any positive ions present in the tube will be attracted towards it. Bombardment of the photocathode by such ions create crystal defects in the semiconductor layer, and sputters away both the cesium/cesium oxide layers and semiconductor layers ultimately destroying the photocathode. Tube life may be rendered unacceptably short by these effects. In addition to degrading tube life, ion feedback is a source of noise. When an ion strikes the photocathode, a large number of secondary electrons are produced. These electrons are then accelerated back into the screen where they show up as a flash of light. These flashes or scintillations are a significant source of noise in proximity focused image intensifiers.
The output screens of light output vacuum tubes are traditionally made of a "phosphor." There are a large number of commercially available phosphor materials with well-known and well described properties. In selecting an output phosphor, the tube designer is interested in selecting one which offers a desired efficiency, color (i.e., spectral characteristics), response time and persistence.
The phosphor output screen is typically made by applying a slurry comprising uniformly sized particles of the phosphor material(s) suspended in a solvent to a glass window which forms a part of the vacuum envelop. Alternatively, one may apply a lacquer to the output window and then brush the dry phosphor powder onto the wet lacquer. While these are relatively easy processes, it is difficult to precisely control the uniformity of the resulting phosphor film.
After the slurry or lacquer is dried, a thin coating of aluminum may be sputtered or evaporated on top of the phosphor to provide electrical contact and to prevent light emitted in the output screen from feeding back to the photocathode. Phosphor screens of the type described have gained almost universal application because they are easy to manufacture, can be selected to offer a desired spectral response and offer high efficiency conversion of electrons to output photons. It should be noted, however, that high efficiency typically comes at a cost of slower response time.
Unfortunately, the traditional phosphor screens, even when coated with a layer of aluminum, release unacceptably high numbers of ions and neutral gas particles when used in a diode tube employing a GaAs or other III-V compound input photocathode. Normal vacuum tube manufacturing techniques such as long tube pump downs and bake outs do not entirely eliminate the problem. It will be recognized that the phosphor screens, comprising a very large number of individual particles, have a very large relative surface area. In night vision tubes using III-V photocathodes, it has heretofore been found necessary to place a physical barrier between the output screen and the photocathode to prevent ions released from the output screen from reaching the photocathode. Even with such a barrier, tube life is limited by the eventual degradation of the photocathode. In certain types of tubes, such as diode photodetectors, a physical barrier is not easily inserted. Furthermore, when used, physical barriers degrade the effective sensitivity of the detector.
Accordingly, it is an object of this invention to provide a light producing output screen having a relatively small surface area for the adsorption of gasses which is particularly useful in tubes employing a III-V semiconductor photocathode.
Another object of this invention is to provide a light producing output screen which is highly uniform.
A further object of this invention is to provide a light producing output screen which is low in noise, is highly efficient and has a very fast response time.
Yet another object of this invention is to provide a light producing output screen comprising a single crystal III-V semiconductor compound as a photon emitter which minimizes losses due to non-radiative recombination of the minority carries at the crystal surface.